Many applications require signal measurement in environments with high ambient noise levels. For example, telescopes which determine the temperature of objects at great distances typically measure small amounts infrared energy in high ambient noise environments. An effective method of filtering a signal in low signal to noise ratio environments is by modulating the measured signal and passing it to a locking amplifier. The measured signal can then be isolated from the ambient noise using synchronous detection techniques.
Optical shutters, also referred to as choppers, are used to modulate optical beams or particle streams. By modulating optical beams or particle streams, optical shutters allow these signals to be measured accurately in environments with high ambient noise.
One type of optical shutter developed to modulate optical beams or particle streams uses a disk with a predetermined pattern of apertures and opaque portions. A motor is used to rotate the disk so that a beam passing through the apertures is interrupted at a desired modulation frequency. Shutters utilizing a rotating disk, however, have limitations which are important to overcome.
Many applications take a longer time to make optical/particle measurements. These applications require the rotating disk shutter to rotate at low speeds. One way to operate a motor at low speeds is to decrease the current to the motor. Motors operating at low current have less torque, so that any intermittent friction within the motor affects the modulation frequency. Changes in the modulating frequency, called "jitter", over a measurement cycle introduce error to the measured signal.
Another source of jitter for rotating disk shutters comes from small differences between the sizes of each aperture on the disk. These differences translate to slight changes in the modulating frequency. Shutter disks which produce very little jitter are often expensive to manufacture.
Finally, some applications require the ability to stop the shutter disk at a point where the beam is passing through an aperture, or an "open" position. Conversely, some applications require the ability to stop the shutter disk at an opaque or "closed" position. This feature is difficult to produce in rotating disk shutters.
For the foregoing reasons, there is a need for an inexpensive optical shutter capable of modulating light beams or particle streams at low frequencies. There is also a need for an inexpensive optical shutter which is jitter free. Finally, there is a need for an inexpensive optical shutter that can be positioned at an open or closed position.